1. Field of the Invention
The present invention relates to a semiconductor laser device using a group III nitride semiconductor.
2. Description of the Prior Art
In general, group III nitride semiconductors of the formula InxGayAlzN (where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1) have wide energy band gaps and high thermal stability, and their band gap widths can be controlled through the adjustment of their composition. For these reasons, their application is being developed in a variety of semiconductor devices such as light-emitting devices and high-temperature devices.
As light-emitting devices, light-emitting diodes (LEDs) that emit light having luminous intensity of the order of a few candelas in a wavelength range of blue to green have already been put to practical use, and laser diodes (LDs) are in the process of being developed for practical use. With respect to laser diodes, from the early stages of their development, the feasibility of using a comparatively easily available insulating substrate, such as sapphire, has been studied.
However, in a device using a sapphire substrate, crystal distortion resulting from a large lattice mismatch between the substrate and the epitaxial layer (about 14% between the sapphire C plane and the GaN crystal) and high-density dislocation defects (108 to 1010 cm−2) introduced into the epitaxial layer have undesirable effects on the device's characteristics such as its working life. Moreover, where sapphire is used as the substrate of a semiconductor laser device, since the substrate and the epitaxial layer have different cleavage planes, it is difficult to obtain satisfactory end surfaces when the end surfaces of the optical resonant cavity are formed by a method relying on cleavage, a common way of forming them.
Attempts have been made to avoid these problems by using as the substrate a material other than sapphire, for example SiC or the like. This, however, does not fundamentally improve the problems associated with the size and availability of the substrate, lattice mismatch, etc.
From the viewpoint of resolving the lattice mismatch between the substrate and the epitaxial layer, reducing crystal defects, and obtaining a satisfactory crystal, the inventors of the present inventions have been developing devices using as their substrate GaN, which, like their epitaxial layer, is a group III nitride semiconductor.
As a result, it is now possible to greatly improve the characteristics of nitride semiconductor laser devices. However, the use of a GaN substrate does not always result in a satisfactory nitride semiconductor laser device. Specifically, it has been found out that, in some such devices, their operation current gradually increases, or their characteristics rapidly deteriorate. Through an intensive study in search of the causes, the inventors of the present invention have found out that there are several methods of producing a GaN substrate, each producing a substrate different in structure and quality, and that the resulting differences affect the layered structure formed on the substrate, greatly influencing the characteristics of a nitride semiconductor laser device.